Voltage converters are used to provide an input voltage based on an output voltage. Voltage converters are used for example in power supplies for a variety of applications, for example mobile phones or other electric appliances. Some voltage converters provide galvanic isolation between an input and any output, e.g. by using a transformer. Examples for such voltage converters include flyback converters. A specific type of flyback converter is an asymmetric pulse width modulation (PWM) half-bridge flyback converter, where an inductor of the converter is essentially being split to form a transformer, such that voltage ratios are multiplied based on a winding ratio of the transformer with an additional advantage of providing galvanic isolation.
In some converters, a primary switch is used on a primary side circuit of the converter, i.e. a side coupled to the input to receive the input voltage. A side coupled to the output to output the output voltage is referred to as secondary side circuit herein, and primary side circuit and secondary side circuit may be galvanically isolated from each other. Switches like such a primary switch are often implemented as transistors, for example metal oxide semiconductor (MOS) field effect transistors (FET). In some implementations, such field effect transistors have parasitic capacitances, also referred to as output capacitances herein. If such a switch is switched with a voltage applied across the transistor, the capacitance is charged, and in the switching event this charge may be lost leading to overall losses of the converter.
Therefore, various approaches have been made to obtain so-called zero voltage switching (ZVS), which means that the primary switch is switched, in particular switched on, while no voltage is applied across the switch (for example between source and drain of a MOSFET switch). Such approaches often involve having an auxiliary switch which injects energy such that zero voltage switching is obtained. Timing of controlling such an auxiliary switch is important to on the one hand obtain zero voltage switching and on the other hand prevent energy losses which may result when such an auxiliary switch is closed for a longer time.
In current approaches, an additional pin of a controller controlling the voltage converter to configure and adjust the on-time of such an additional switch is used. Such additional pins increase production costs. Furthermore, current approaches may not provide an optimum timing of an auxiliary switch.